1. Field of the Invention
The present invention relates to surface-mount positive coefficient thermistors for use in, for example, protecting exchanges and telephones from overcurrents.
2. Description of the Related Art
FIGS. 4A and 4B show a surface-mount positive coefficient thermistor 1 which includes a surface-mount type disc positive coefficient thermistor element 2. A pair of electrodes 3 is provided on two main surfaces of the disc positive coefficient thermistor element 2. Each electrode 3 includes a nickel-plated underlayer 3a and a silver-baked outermost layer 3b. Furthermore, one end 4a of a pair of metal plate terminals 4 are connected to each outermost layer 3b via solder 5. The metal terminals 4 are made of nickel silver or stainless steel and are plated with tin. The other end of each metal terminal 4 is connected to an electric circuit (not shown) on a board with solder.
In the surface-mount positive coefficient thermistor 1, the metal terminal 4 and the electrode 3 are connected by the solder 5 by a reflow soldering process which includes applying a solder paste between one end 4a of the metal terminal 4 and the electrode 3, heating to melt the paste 5 in a reflow furnace, and then cooling the melt to bond the electrode 3 to the metal terminal 4.
In the metal terminal 4 which is made of a material having a low thermal conductivity, such as nickel silver or stainless steel, heat that is applied to one end 4a during soldering does not significantly transfer to the other end 4b. Since the other end 4b is not oxidized, the solderability to the board does not substantially decrease.
The metal terminal 4 made of such a low-thermal conductivity material, however, precludes heat transfer from the end 4a to the solder 5 and the solder 5 is difficult to melt.
Thus, the solder 5 does not completely melt during a short heating time, resulting in low bonding strength between the metal terminal 4 and the electrode 3. If heat is applied for a sufficiently long time to completely melt the solder 5, the end 4a of the metal terminal 4 is discolored and the plated-tin is oxidized, resulting in substantial deterioration of wettability of the solder when a board is mounted.
Referring to FIG. 5, horizontal arrows represent thermal conduction in the solder 5. Applied heat does not rapidly transfer to the center of the solder 5, thus precluding uniform heating.
In order to overcome the above-described problems, preferred embodiments of the present invention provide a metal terminal that is configured to allow adequate and uniform heating when the metal terminal is soldered to an electrode, and a method for making a surface-mount positive coefficient thermistor including such a novel metal terminal.
According to a preferred embodiment of the present invention, a surface-mount positive coefficient thermistor includes a plate-like positive coefficient thermistor element, a pair of electrodes on the two main surfaces of the positive coefficient thermistor element, each electrode including at least two layers, a pair of metal terminals each having a cutout at one end region thereof connected to the corresponding electrode, the metal terminals being made of a metal having a low thermal conductivity, the end region of each metal terminal being connected to the outermost layer of the corresponding electrode, the outermost layer of the electrode being exposed at the cutout, and solder connecting the end region of the corresponding metal terminal to the outermost layer of the corresponding electrode.
Preferably, each of the electrodes includes a first layer of nickel as the primary component and a second layer of silver as the primary component, the area of the second layer is less than that of the first layer such that the peripheral edge of the second layer is spaced from the peripheral edge of the corresponding main surface.
Preferably, the metal terminal includes at least one material selected from stainless steel and silver nickel.
Preferably, the metal terminals are subjected to a plating treatment to enhance solderability.
Preferably, the solder overflows the cutout onto the outer surface of the metal terminal.
According to another preferred embodiment of the present invention, a method for making a surface-mount positive coefficient thermistor includes the steps of preparing a plate-like positive coefficient thermistor element, forming electrodes on the two main surfaces of the positive coefficient thermistor element, preparing a pair of metal terminals, forming a cutout at an end of each of the metal terminals, and applying solder between the respective ends of the metal terminals and the respective electrodes, and then melting by heating the solder to connect the metal terminals to the respective electrodes.
Preferably, the solder is melted by heat from a near infrared heat beam.
In a surface-mount positive coefficient thermistor, a large impact is applied from the top during mounting as compared with a lead type positive coefficient thermistor. Thus, secure bonding between the thermistor element and the metal terminals is required. According to preferred embodiments of the present invention, the cutout provided at one end of each metal terminal facilitates direct heating of the solder provided under the metal terminal. Thus, the solder is adequately and uniformly heated and melted. Furthermore, the solder overflows the cutout into the outer surface of the metal terminal, resulting in greatly improved bonding between the metal terminal and the electrode.
Furthermore, in the surface-mount positive coefficient thermistor which is exposed to a higher temperature environment when it is mounted onto a circuit board, a solder having a higher melting point must be used for bonding the metal terminal with the electrode. In preferred embodiments of the present invention, a near infrared heat beam is used to heat the solder rapidly to a high temperature. Since the metal terminal is not substantially oxidized, the solder has satisfactory wettability when the thermistor is mounted onto a circuit board.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.